A tyre has a structure composed by layering together various materials, such as rubber, chemical fibers, steel cords, and the like. If there is an irregular portion in this laminated structure, then a protuberance (convex portion) known as a “bulge”, or a recess (concave portion) known as a “dent” or “depression”, occurs in a portion which has relatively weak pressure resistance, when the tyre is filled with air. Upon inspection, a tyre in which a shape defect, such as a bulge or dent of this kind, has occurred must be excluded from shipment, due to safety problems and problems of external appearance.
Therefore, in a final step of tyre manufacturing (an inspecting step after vulcanization of the tyre material), the tyre surfaces, and in particular the side wall surfaces of the tyre, are inspected for irregular shape defects. Indicator marks (marks constituted by normal unevennesses) which indicate the model and size of the product, and the makers logo, etc., are formed in the sidewall surfaces of a tyre. Therefore, in a shape defect inspection process for a sidewall surface, it is necessary to ensure that these indicator marks are not mistakenly detected as shape defects.
Conventionally, an inspection for irregular shape defects of this kind has been carried out manually, by visual inspection and tactile inspection, but in recent years, automated technology, such as laser ranging sensors, three-dimensional shape measurement devices and camera-based image inspection, and the like, and inspection technology which is not affected by marks formed by normal unevennesses, has been incorporated into the field.
For example, Patent Document 1 discloses a tyre shape detection device which detects a surface shape of a tyre by capturing an image of a line light beam irradiated onto the surface of the tyre while the tyre rotates relatively, and performing shape detection using a light-section method, on the basis of the captured image. This device comprises line light beam irradiating means for irradiating a plurality of line light beams in continuous fashion from a direction that is different to a detection height direction in one light-section line, so as to form the one light-section line on the surface of the tyre, and imaging means for capturing images in a direction in which respective main light beams of the plurality of line light beams irradiated onto the surface of the tyre are reflected regularly with respect to the surface of the tyre.
In particular, this tyre shape detection device detects a tyre surface shape by irradiating a plurality of line light beams in continuous fashion onto the tyre surface and capturing images of the plurality of line light beams thus irradiated.
Furthermore, Patent Document 2 discloses a method of inspecting a three-dimensional shape of one or more figures formed by unevennesses in a tyre surface. This method comprises a step of acquiring unevenness distribution data by measuring an unevenness height in relation to various surface area elements including these figures in a prescribed tyre surface region, a step of identifying a tyre surface portion corresponding to a model figure, in the tyre surface region, on the basis of three-dimensional shape data of the model figure which is previously prepared as a template of the figure and the acquired unevenness distribution data, and a step of determining a degree of matching between the unevenness distribution data of the identified tyre surface portion and the three-dimensional data of the model figure, in respect of each figure, and judging the acceptance or rejection of the three-dimensional shape of the figure of the basis of this degree of matching.
In particular, this method of inspecting tyre unevenness figures is a technique for inspecting defects by calculating a degree of matching between three-dimensional unevenness distribution data obtained by irradiating sheet light onto a tyre surface and three-dimensional shape data of a model figure created from CAD data. In this technology, the suitability or unsuitability of the normal uneven figures (text, etc.) themselves is judged, and therefore a model figure previously prepared as a template of a normal uneven figure is used as teaching data. The template is created from tyre CAD data or mold CAD data.
Patent Document 1: Japanese Patent Application Laid-open No. 2008-221896
Patent Document 2: Japanese Patent Application Laid-open No. 2005-331274
In the tyre shape detection device according to Patent Document 1, it is possible to detect the surface shape of a tyre by a light-section method, and therefore the shape of unevennesses in the tyre surface can be detected. However, it is not possible to tell whether a detected shape of unevennesses in the tyre surface is a normal figure which is formed in the tyre surface, or a defect. Moreover, if there is a defect at the position of a normal figure, then it becomes even more difficult to detect that defect.
Therefore, by creating teaching data (reference data) using the tyre CAD data or mold CAD data as disclosed in Patent Document 2, it might be possible to obtain values which are not affected by deformation or defects in the tyre, and to avoid the difficulties of the technology disclosed in Patent Document 1. However, since tyres are products made of rubber and since tyres are inspected in an inflated state in the tyre shape inspection which is the subject of the invention in Patent Document 1, then there is a large amount of deformation in the tyre with respect to the CAD data. For this reason, an enormous volume of calculations and computations is required just to match the coordinates of the CAD data with the corresponding coordinates on the tyre, and therefore the method in Patent Document 2 is difficult to apply practically.
Moreover, a method which uses actually measured tyre height data as teaching data can readily be inferred from the method using CAD data which is disclosed in Patent Document 2. According to this method, it is possible to acquire actual height data, in a simple fashion.
However, in this case, the tyre height image which is used as teaching data must contain only normal uneven figures, and must be completely free of irregular defects (bulges/dents) which are the object of detection, or height variations caused by “runout” components, which are large undulating deformations in the tyre circumference direction. If height image data which contains irregular defects that are the object of detection, or runout components, is used as teaching data, then although normal uneven marks, such as text, are planarized (excluded) by difference processing during on-line inspection, the irregular defects and runout components present in the teaching data will be transferred to the height image under inspection, and hence height image data of this kind cannot be used as teaching data for inspection. Furthermore, it is not practicable to manufacture a perfectly smooth tyre which is free of runout components, for the express purpose of registering teaching data.